@egreg: There is no line 67 because this code is a part of the code. Replace line 67 with \begin{equation}, right after \begin{subequations}
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ThanosSep 10 '12 at 15:20

Please try removing packages until the error continues to be raised. What TeX distribution are you using?
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egregSep 10 '12 at 15:30

I am using LaTeX...I don't think that it is important, but I am using subfiles...I tried to do the same thing without subfiles, but the problem remains...I'll try removing packages...
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ThanosSep 10 '12 at 15:38

It is working...let's say... The problem is that I am writing a book. These equations are part of a chapter, which contains more equations. So if I add later on another equation, it will not be numbered correctly, let alone I am suppossed to have a number like 1.1a and not 1a
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ThanosSep 10 '12 at 9:41

You can use this one if you want to have an output 1.1a \renewcommand{\theequation}{1.1\alph{equation}} \setcounter{equation}{0}
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jeecabzSep 10 '12 at 10:11

But again the problem remains. If I want to add another equation, it will be numbered 1.1b and not 1.2. Somehow I have to reset the counter, but I want to avoid 1.1.a...
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ThanosSep 10 '12 at 15:27